1. Field of the Art
The present invention relates to a bearing device which has a liquid hydrodynamic bearing as a radial bearing, and a gas hydrodynamic bearing as a thrust bearing, and which supports a rotating object rotating at high speed. More particularly, the present invention relates to a bearing device suitable for a spindle motor used for driving a hard disk driver (hereinafter referred to as "HDD"), a laser beam printer driver (hereinafter referred to as "LBP"), a rotating drum device for a video system, etc., in which excellent rotating performance is required irrespective of the position of the motor.
2. Prior Art
With the achievement of high storage capacity and high-speed rotating performance in recent HDDs, the spindle motors which drive them have accordingly been demanded to have high performance. That is, it has been demanded to improve durability, cleanness and high-speed rotating performance and to minimize vibrations during rotation irrespective of the position of the motor when used, so as to be even more suitable for such HHDs.
FIG. 8 is a sectional view showing the structure of a conventional spindle motor for HDD. In FIG. 8, a mount (base) 31 has a support shaft (spindle) 32 stood on the central portion thereof. An annular thrust plate 33 is secured to the mount 31, and a radial cylindrical member 34 is concentrically secured to the support shaft 32. A plurality of circumferentially equally spaced stator coils 35 are secured to the support shaft 32 above the radial cylindrical member 34. A support member (hub) 36 having a cap-shaped configuration is provided on the support shaft 32. The ceiling portion at the upper end of the support member 36 is loosely fitted on the upper end portion of the support shaft 32. The support member 36 has an annular bearing member (serving as both radial and thrust sleeves) 37 secured to the lower end portion thereof. The annular member 37 has an L-shaped cross-sectional configuration. The lower end surface of the bearing member 37 faces the upper surface of the thrust plate 33, while the inner peripheral surface of the annular member 37 faces the outer peripheral surface of the radial cylindrical member 34. Either the lower end surface of the bearing member 37 or the upper surface of the thrust plate 33 is formed with spiral grooves for generating dynamic pressure in the thrust direction. Either the inner peripheral surface of the bearing member 37 or the outer peripheral surface of the radial bearing member 34 is formed with herringbone-shaped grooves for generating dynamic pressure in the radial direction.
A plurality of circumferentially equally spaced rotor magnets 38 are secured to the inner periphery of the support member 36 in opposing relation to the stator coils 35. As the stator coils 35 are sequentially supplied with an electric current, the support member 36 having the rotor magnets 38 begins to rotate, and consequently a pneumatic dynamic pressure is generated between the upper surface of the thrust plate 33 and the lower end surface of the bearing member 37, and a pneumatic dynamic pressure is similarly generated between the outer peripheral surface of the radial bearing member 34 and the inner peripheral surface of the bearing member 37. Thus, the upper surface of the thrust plate 33 and the lower end surface of the bearing member 37 constitute a thrust bearing, while the outer peripheral surface of the radial bearing member 34 and the inner peripheral surface of the bearing member 37 constitute a radial bearing. The support member 36 rotates while being supported by the thrust and radial bearings.
However, the above-described spindle motor suffers from the problem that, when it is operated in a horizontal position (i.e., in a direction in which the direction of gravity is perpendicular to the shaft of the motor), a moment in the radial direction is generated due to the gravity of the rotor, causing the axis of the rotor to be inclined with respect to the radial bearing, which results in an increase in the imbalance of radial magnetic force acting between the rotor magnets 38 and the stator coils 35, and in this state a rotor member is brought into local contact with the bearing.
Further, since the thrust plate and the radial bearing member are comprised of discrete members and are independently positioned on the mount, it is difficult to achieve the required perpendicularity between them at the time of assembly.
In addition, at the time of starting and stopping the spindle motor, the radial cylindrical member and the radial sleeve come in direct sliding contact with each other and hence wear is caused in the process of repeating start and stop.
In addition, the number of revolutions of scanner motors used in LBP is increasing year by year, so that it has become difficult for the conventional bearings to cope with the high rotational speed of these motors.